A number of barriers impede anti-tumor activity of drugs, antibodies, and nanoparticles including: sieving properties of the extracellular tumor matrix, high intra-tumoral hydrostatic pressures and the low volume of blood flow that metastatic tumors receive. The combination of these factors restrict the delivery of high quantities of therapeutic agents into tumors. The tumor homing of macrophages is well documented and recent advances enable the ex-vivo production of self-renewable, syngeneic and fully differentiated normal macrophages (SSM). We propose to generate and then modify SSM from BALB/c mice via gene insertions or drug-loaded nanoparticles (liposomes) and then re-inject the armed SSM back into BALB/c mice with orthotopic 4T1 breast tumors. We will investigate factors that affect the time course and percent of the SSM dose that migrates into the 4T1-luc primary tumor as well as into metastatic sites distal to the mammary fat pad such as the brain or lung, identified using the luciferase reporter in the 4T1 line. We will load agents into the SSM that maintain macrophages in the M1 phenotype to enhance their anti-tumor activity. We will investigate the effect of the SSM and the agents they deliver on the phenotype of the tumor resident macrophages and determine if tumor resident macrophages can be returned to a M1 phenotype by IL12 and Interferon gamma secreted by the SSM. Finally, we will determine the anti-tumor activity of armed SSM against the 4T1 tumor in BALB/c mice. Three specific aims will be aggressively pursued. Aim 1. Devise liposome formulations and culture conditions for highly efficient in vitro loading of nanoparticles into RAW309 Cr1 cells as a macrophage prototype. Evaluate the rate, extent and tumor distribution of liposome-loaded macrophages in the 4T1 murine breast tumor model as a function of the number of macrophages injected. Completion of this aim will guide the loading, dosing and frequency of doses in aim 3. Aim 2. Generate and characterize self-renewing syngeneic macrophages from BALB/c derived normal bone marrow cells by genetic removal of c-Maf and MafB function. We will also modify the SSM with a red reporter protein and yeast cytosine deaminase or interferon gamma/IL12. Aim 3. Evaluate the tumor distribution and therapeutic activity of SSM created in aim 2 and loaded with drug nanocarriers or inducible proteins such as yeast cytosine deaminase, or Interferon gamma/IL12 in BALB/c 4T1 breast cancer model. Our studies will generate a quantitative understanding of the factors that control the trafficking of macrophages into tumors as well as the potential for this modality to control tumor growth and metastases. Success in this research could enable the development of new autologous macrophage cell-based therapies that greatly increase therapeutic agent delivery into and throughout tumors. It could also provide an effective means to re-educate tumor-associated type M2 macrophages to become tumor killers. If effective, armed SSM could also provide a personalized cancer treatment to eradicate metastatic tumors that cannot be achieved by currently available approaches.